Summary/Abstract for Overall Program The long-term objective of this program project is to determine how herpes simplex virus 1 (HSV-1) can switch between a highly active lytic infection that produces infectious virus and a more silent latent infection. There are several hypotheses regarding mechanisms that can tilt the balance in favor of lytic infection or latent infection. These mechanisms include viral gene products that affect the chromatin status of the viral genome, post-transcriptional mechanisms that can affect viral and host gene expression, and immune responses and viral gene products that combat those responses. None of these mechanisms is solely responsible for the lytic/latent balance, and each of these mechanisms is highly likely to be connected. For example, immune responses can affect viral gene expression, and post-transcriptional mechanisms can repress the expression of viral gene products that affect chromatin status and combat immune responses. Thus, an integrated approach to the lytic/latent balance is needed. In this Program Project proposal, three senior herpesvirologists with complementary areas of expertise will conduct a series of highly collaborative studies to investigate these mechanisms in three intertwined projects with the aid of three cores. Project 1 will study (including collaborative studies with Project 2) how viral latency- associated transcripts (LATs), the viral protein ICP0, and the host protein CTCF effect a chromatin configuration poised for reactivation during establishment and maintenance of latency in vivo, and with Project 3, in cultured mouse neurons, and in human neurons. Project 2 will focus on how post-transcriptional regulatory mechanisms can repress lytic gene expression and affect chromatin status (with Project 1) and a viral gene product that counteracts immunity (with Projects 3 and 1) and contribute to latency, and a virus block to nuclear export of miRNAs (with Project 1) and the targets of viral miRNAs in cultured cells including mouse and human neurons (Projects 3 and 1). Project 3 will use an in vitro model of latency using cultured neurons from mice, including genetically altered strains, and viral mutants (some from Projects 1 and 2) to test roles of neuron-specific autophagosomes, viral proteins that counter immunity, and antibodies in the nervous system (including those generated by a vaccine from Project 1) in control of viral replication, latency, reactivation, and, gene expression.